The present invention relates to a wireless network system for performing a bidirectional transmission of data through a communication satellite, for example, between a master station and a plurality of slave stations, and to a wireless communication apparatus used as a master station in this system.
In a satellite communication system, a connecting aspect for connecting in a multiplex manner between a plurality of earth stations includes a mesh type and a star type.
In the mesh type system, for example, as shown in FIG. 1, slave stations B3 and Bn in an idle state among a plurality of slave stations B1 to Bn are connected to a master station A through a common line C and, slave stations B1 and B2 in an information communication state are directly connected to slave stations B5 and B4 for the communication object through a communication line T.
The connection between the slave stations is controlled by a line control device provided in the master station A. FIG. 2 shows an embodiment of a connection sequence of the same. Accordingly, the slave station (a transmission station) which is going to sends a communication request signal to the master station A through the common line C. When the master station A receives the communication request signal, the master station A sends an incoming call instruction signal to the opposite slave station (a reception station) through the common line C and wait for returning of an incoming call response signal from the reception station. Further, when the incoming call response signal is returned, the master station A sends a line allocation signal to the transmission station and the reception station through the common line C. The transmission station and the reception station captures the communication line allocated by the line allocation signal, and thereafter, they become in a communication state. When the communication is terminated, a line deallocation information signal is transmitted to the master station A from each of the transmission station and the reception station through the common line C, and the master station confirms a deallocation of the line by the signal and transmits the deallocation information signal to each of the slave stations in the transmission side and the reception side. Accordingly, a demand assigned multiple access method (DAMA) is employed for an access method.
The mesh type system has an advantage that the communication can be directly performed between the slave stations, however, since each of the slave stations has to be provided with a transmitting/receiving equipment capable of performing a communication with the slave station on the smallest scale among the system, there is a disadvantage that a size of the slave station is made large and it is hard to structure an economical system. Further, in the case of performing a broadcast communication, it is necessary to prepare an exclusive line for the broadcast in addition to the common line.
In comparison with this, in the star type system, for example, as shown in FIG. 3, each of the slave stations B1 to Bn is connected to the master station A, and all the communication between the slave stations is performed through the master station A. Accordingly, all of each of the slave stations B1 to Bn has to do is to perform a communication with the master station A, so that the slave station can be made simple and compact by making the master station A share a main portion of the communication equipment, thereby structuring a system more economical than the mesh type system.
In this case, in the conventional star type system, as the line for connecting between the master station A and each of the slave stations B1 to Bn, an inbound line and an outbound line having a different frequency are prepared. Among them, the outbound line is used in the case that the master station A transmits the data to the slave stations B1 to Bn. Since this transmission is performed under a control of the master station A, a conflict and a collision are not generated.
In the inbound line, since a plurality of slave stations B1 to Bn independently use a common radio frequency, there is a case that the conflict and the collision are generated. Then, in the conventional art, for example, the following access methods are employed.
a. Random Access Method
This is a method that each of the slave stations uses the line in such a manner that first come, first served, and in this method, a control sequence is simple. However, the line can be used only to a physical level of 1/e (in which e is a base of a natural logarithm) as a use efficiency (a throughput) of the line, so that there is much uselessness. Further, in the case that a traffic is increased, the line becomes an overcrowding state, so that it is necessary to sufficiently take the overcrowding into consideration in a step of a system design.
b. Random Access and Line Reservation Method
In the case that an amount of the transmission data is a little, only the random access method is used. In this method, in the case of transmitting the relatively large amount of data, after the communication is started by using the random access method, a signal transmission in all the other slave stations having the frequency channel in common is stopped and the specified slave station exclusively has a right for using the frequency channel. Accordingly, there is a case that an immediate performance of the communication can not follow an increase of the traffic amount.
c. Polling Method
This is a system that polling is performed to each of the slave stations from the master station at regular intervals, and each of the slave stations transmits a signal in accordance with the polling, in which the use efficiency of the line can be increased. However, there is a disadvantage that a transmission delay of the data to the master station from the slave station becomes large.
As mentioned above, in the conventional wireless network system, there have been the following problems Accordingly, in the mesh type system, since it is necessary to secure a communication with respect to the slave station having the worst condition among the system, it can not be avoided to make the transmission/receiving equipment of the slave station large, so the the economical system can not be structured. On the other hand, in the star type system, at a time of accessing the inbound wireless line, there are generated the problems that the throughput is insufficient, the flexibility with respect to the change of the traffic amount, the transmission delay is large a the like.